The ability to detect defects, such as holes or voids, e.g., airlines, in optical waveguide fibers is of central importance in providing high quality fibers and in devising manufacturing techniques which minimize the occurrence of such defects. Holes or voids typically occur at the center of a fiber (on-center holes), although they can be located anywhere in the fiber cross-section (off-center holes).
In the past, holes have been detected during the drawing of fibers as part of the procedures used to measure fiber diameter. Specifically, fiber diameter has been determined using the optical technique described in U.S. Pat. Nos. 3,982,816 and 4,067,651 to Lawrence Watkins. The basic components of the Watkins system are schematically illustrated in FIG. 1.
As shown therein, optical waveguide fiber 13, whose cross-section has been greatly expanded for purposes of illustration, is transversely illuminated by light 15 of sufficient spatial coherence and monochromaticity to create a discernible interference pattern in the far field, that interference pattern being created by the superposition of light reflected from the fiber surface 17 and light refracted through the fiber body 13. In practice, a laser, e.g., a HeNe laser, is the preferred light source because of its wavelength stability. The following discussion is thus in terms of a laser light source, it being understood that other light sources having sufficient spatial coherence and monochromaticity can be used if desired.
As explained in the Watkins patents, in the far field, this reflected and refracted light interferes to form fringe pattern 19. For an optical waveguide fiber having a core and a cladding, the fringe pattern will in general be a function of the wavelength of the incident light and of the indices of refraction and the diameters of both the core and the cladding. However, as shown by Watkins, if the core/clad ratio is not too large and if the fringe pattern is examined at sufficiently large angles, e.g., above about .+-.50.degree. in FIG. 1 for core/clad ratios of less than about 0.5, the pattern will depend almost exclusively on the diameter and index of refraction of the cladding.
Accordingly, if the index of refraction (n) of the cladding is known, the outside diameter (d) of the fiber can be determined by analyzing the fringe pattern. Specifically, the diameter can be approximated with good precision by counting the number of full and partial fringes (N) between two angles (.theta..sub.a and .theta..sub.b) and then using the following equations to calculate d: EQU E(.theta..sub.a)=sin (.theta..sub.a /2)+[n.sup.2 +1-2n cos (.theta..sub.a /2)].sup.1/2 ( 1) EQU E(.theta..sub.b)=sin (.theta..sub.b /2)+[n.sup.2 +1-2n cos (.theta..sub.b /2)].sup.1/2 ( 2) EQU d=N.lambda./[E(.theta..sub.b)-E(.theta..sub.a)] (3)
where .lambda. is the wavelength of the laser light used to illuminate the fiber. Note that in equation 3, there is a direct relationship between diameter and fringe count. In practice, given an invariant clad index and an invariant wavelength, one can calibrate the system with an empirical constant which, when multiplied by the number of fringes, gives the diameter.
A typical fringe pattern in the range from +50.degree. to +70.degree. for a 125 micron single mode fiber is shown in FIG. 3(a). Approximately 62 fringes appear in this 20.degree. range, which is as predicted by the Watkins model.
The effect of a 20 micron, on-center hole on the pattern of FIG. 3(a) is shown in FIG. 3(b). As can be seen in this figure, the presence of the hole results in missing (fewer) fringes. In the past, the change in fringe pattern between FIGS. 3(a) and 3(b) has been used during the fiber drawing process to detect holes.
Specifically, when a fiber is being drawn at or near its target diameter, the location of each fringe in the interference pattern is predictable. Using this fact, holes have been detected by watching for a missing sequence of fringes of a prescribed, user-settable length, e.g., two missing fringes in a row.
Although this technique has worked reasonably well in practice, it has suffered from a number of problems.
First, small holes result in the loss of only a few fringes and thus can be easily missed. Accordingly, fiber may be considered acceptable when in fact it contains holes. Also, holes tend to start small, grow larger, and then diminish in size. The inability to detect small holes means that the beginning and end of a hole's life cycle cannot be seen. Accordingly, long lengths of fiber on either side of the portion of a fiber where a hole is detected must be discarded to insure that the entire hole is removed.
Second, although the hole detector may miss a small hole, the reduced number of fringes produced by such a hole will be detected by the overall control system and interpreted as a reduction in the fiber's diameter. The response will generally be to inappropriately increase the fiber's diameter. Accordingly, not only might fiber be sold with small holes, but the diameter of the fiber may also be off nominal.
Third, even if a hole does grow large enough to be detected, thus alleviating the concern of sending unacceptable product to a customer, the resultant diameter mismeasure is s gross that the control system is significantly disturbed and takes a substantial amount of time to restabilize.
Techniques for detecting defects in fibers and/or for dealing with the effects of defects on fiber diameter measurements can be found in Smithgall, Sr. U.S. Pat. No. 4,046,536 (analysis of fringe counts in the presence of "dropouts" resulting from faults in the fiber); Bailey et al. U.S. Pat. No. 4,924,087 (detection of fiber defects using light scattered out of the plane of the basic diffraction pattern); Douklias U.S. Pat. No. 4,501,492 (detection of fiber defects and testing of fiber diameters using a spatial filter prepared using diffracted/scattered light from a defect-free fiber); Maillard et al. U.S. Pat. No. 4,541,856 (use of "diffused" light to detect bubbles, blisters, and solid particles in a stream of molten glass); and Young, II U.S. Pat. No. 4,136,961 (detection of defects in glass blanks by rotating the blank through a thin beam of light).
The use of fast Fourier transforms (FFTs) to analyze fringe patterns in order to determine fiber diameters is discussed in an article by Mustafa Abushagur and Nicholas George entitled "Measurement of Optical Fiber Diameter Using the Fast Fourier Transform," Applied Optics. Jun. 15, 1980, vol. 19, no. 12, 2031-2033. The article contains no disclosure or suggestion of the use of fast Fourier transforms to detect defects in fibers.